Time division multiple access (TDMA) communication systems are known. Such systems typically include one or more base stations through which portable wireless devices (e.g., personal digital assistants (PDAs), cell phones, etc.) can access other devices or wireline communication services.
In order to gain access through a base station, the portable device may first monitor one or more control channels to detect a signal from a base station. In order to facilitate access, the base station may transmit an identification signal on the control channel.
In TDMA wireless systems, it is important for the receiver to acquire timing and frequency synchronization in order to correctly demodulate the received data signal and to accurately measure the carrier to interference ratio. Both of these objectives can be achieved by a transmitting device embedding a known stream of symbols in the transmitted signal. These known symbols are often referred to as reference symbols.
The known stream of embedded symbols may include at least some SYNCH and/or PILOT symbols. SYNCH symbols are used primarily for timing estimation and PILOT symbols are used primarily for frequency estimation.
The receiver can process the SYNCH symbols to obtain time synchronization by correlating the received SYNCH symbols with known values for the SYNCH symbols within a repeating frame structure. Once time synchronization has been achieved, the receiver may use the PILOT symbols to estimate a frequency offset.
The frequency offset is the difference between the received signal carrier frequency and the locally generated reference frequency. The frequency offset may be caused by any of a number of different factors internal to the portable devices (e.g., clock offset, Doppler effects, asynchronous clock drift, etc.).
For reference symbols (e.g., pilot symbols) with a repetition frequency of Fp, the maximum frequency offset that can be estimated using conventional methods (without introducing aliasing) is Fp/2 (i.e., the Nyquist frequency of the pilot symbol repetition rate). If the frequency offset is greater than Fp/2, then conventional methods may incorrectly estimate the frequency, and carrier to interference (C/I) measurements will be severely degraded resulting in improper channel selection, demodulation errors and high symbol error rates. Accordingly, a need exists for better methods of calculating C/I values.